Rosamund Strode, who has died aged 82, was Benjamin Britten's music assistant from 1964 until his death in 1976, and subsequently keeper of manuscripts at the Britten-Pears library in Aldeburgh until her retirement in 1992. Fiercely protective of Britten's privacy, she was self-effacing and modest about her own achievements, which were considerable. Her death deprives Britten scholarship of one of those closest to the composer, and although she was always generous in sharing her knowledge, much that was known only to her is now lost.

One phase of her life's work came to an end in 1976, but the establishment of the Red House library, by Britten and his partner Peter Pears, meant that Rosamund's role continued to be indispensable. As the founding archivist of the Britten-Pears foundation, she instituted the guidelines by which the extraordinary collection of music manuscripts, letters and photographs – the most comprehensive of any composer – was to be conserved and safeguarded. She could have clung on to a role as guardian of the Britten flame, but she retired gracefully, happy that she could pass on to her successors much of the knowledge that she had acquired, and ungrudgingly gave advice without any claim of ownership.

Retirement, though, was notional, as she remained central to Aldeburgh affairs. She was able to find more time for writing, although her attention to detail meant that this always proceeded slowly. Even a letter to a friend was the work of a perfectionist. She contributed a number of acute essays to Britten studies, notably for the facsimile edition of Peter Grimes in 1995, and was joint author, with Christopher Grogan, of the major study Imogen Holst: A Life in Music (2007). She was active as chair of the Holst foundation for many years, coming to her last meeting in fine spirits in December of last year.

Born in Amersham, Buckinghamshire, Rosamund was the third child of Maurice Strode and Nancy Gotch (granddaughter of the Victorian painter John Callcott Horsley). She recalled her mother holding her up to a window at the age of five to see Gustav Holst, for many years director of music at St Paul's girls' school, west London, at work in his soundproofed studio. She was educated at St Mary's school in Calne, Wiltshire, and studied viola and singing at the Royal College of Music, where she had composition lessons from Ralph Vaughan Williams. In 1948 she was introduced to Holst's daughter, Imogen, who was then teaching at Dartington Hall, and was immediately recruited by her as an assistant. It was at Dartington that she first met Britten, playing viola alongside him at a rehearsal.

She had intended to train as a county music organiser, but her experience at Dartington turned her back towards a singing career, and in 1952 she joined the choir that Imogen had founded in London, which was to become the Purcell Singers. A fine, light soprano, she performed frequently at Aldeburgh, where Imogen had become resident as assistant to Britten, taking the solo part in the first performance of an early choral piece of Britten's which he revised for the 1955 Aldeburgh festival. Increasingly, she was called upon by Imogen to act as her assistant in turn, researching early works in London, and in 1956 helping with the huge score of Britten's ballet The Prince of the Pagodas.

Rosamund became a part-time assistant to the festival, and as her work for Britten increased, eventually came to work full time for him in 1964, when Imogen decided that she must devote more time to her own interests, in particular looking after her father's legacy. One of Rosamund's first tasks was to order and catalogue the huge range of Britten's childhood music, which gave her an unparalleled insight into the composer's working methods.

She had an elegant and clear musical hand, which, coupled with her care for accuracy, was crucial to her main roles. These were to prepare fair copies of Britten's chamber music scores, to lay out the preparatory full scores of his operas – most importantly for Death in Venice – and to edit his music for publication. She worked in a tiny office in the courtyard of the Red House, always piled high with music, often obscuring her from sight. Her many other tasks included page-turning for Britten when he played the piano, and acting as general factotum during the intensive period of the Aldeburgh festival.

Rosamund was an unforgettable and essential part of Aldeburgh, her rosy cheeks and ready smile belying a seriousness of purpose that could take people by surprise. At the same time, she was disparaging of her musical abilities, claiming (quite wrongly) that many others could have done what she did. She could be impatient, but her first instinct was to make sure that people did not remain in ignorance. She inherited from Imogen Holst a mission to explain, and her legacy will continue to be found in the writings and memories of the many musicians and scholars who came to learn at her feet.

She is survived by two of her three brothers.

• Rosamund Strode, musician, editor and administrator, born 18 May 1927; died 26 March 2010